The invention relates to a retention system used to prevent axial movement of a turbine bucket dovetail in a corresponding dovetail slot in a turbine rotor wheel, and more specifically, to techniques for preventing circumferential rotation of the axial retention system. This retention system typically takes the form of a lockwire within an annular slot or groove in the turbine rotor wheel.
In conventional turbine and/or turbine compressor components, buckets (or blades, or airfoils) are held in a rotor wheel by means of a slotted connection, e.g., a so-called “fir tree” or “Christmas tree” arrangement where an inwardly-tapered male connector portion at the radially inner end of the bucket is received in a complementary female slot in the rotor wheel. Such connections are also generically referred to as “dovetail” connections, embracing various complementary shapes which lock the buckets to the wheel in the radial and circumferential directions so as to accommodate the high centrifugal forces generated by rotation of the turbine rotor.
The fit between the blade dovetail and the dovetail slot is sufficiently loose to allow for assembly and tolerances. Centrifugal loading above a certain threshold speed effectively locks up the bucket in the wheel due to the contact forces and friction. However, operation at low speed, during which the blades are able to rock inside the dovetail, can have the tendency to make the blade move along the dovetail in the absence of axial retention. If the blade is not properly retained, the eventual likely outcome is a collision with neighboring stationary components. Before such collision can take place however, the axial movement along the dovetail could effectively block cooling flow into the blade. In the absence of the cooling flow, oxidation erosion will wear away the leading edge of the blade. An additional consequence, therefore, is unplanned machine down-time and maintenance resulting from varying degrees of machine performance deterioration up to blade separation and resulting collateral or domestic object damage.
In accordance with usual design practice, the buckets or blades are prevented from moving axially in the dovetail slots provided in the rotor wheel by a retention device, hereafter called a “lockwire”, passing through an annular slot formed in the radially outer periphery of the wheel and passing through circumferentially-aligned slots in the dovetail portions of the respective buckets. The free ends of the wire are shaped so that they come together at an overlapped joint, thus allowing for minor changes in length and diameter of the lockwire as the rotor wheel, rotor wheel slots and buckets expand and contract during transient periods. The lockwire is held in place by the radial spring force stemming from installation of a relatively larger-diameter lockwire in a relatively smaller-diameter annular slot, and pins mounted in the turbine wheel, radially inwardly of the lockwire. It has been discovered that rotation of the lockwire within the annular slot in the rotor wheel (which occurs over time) can cause the free ends of the lockwire to separate at the overlap joint so that one end of the lockwire may engage a pin and bend downwardly (radially inwardly) below the pin and, thus permit the lockwire to escape the annular slot.
Without the lockwire, the airfoils are free to travel axially along the dovetail slots, creating the potential for excessive wear and interference as mentioned above. In addition, this is especially consequential in first and second stage buckets that rely on holes in the base of the bucket to provide internal cooling. When these holes are blocked due to axial movement of the bucket, cooling air cannot reach the target area and the bucket can quickly oxidize along the leading edge.
There remains a need for a reliable technique for preventing circumferential rotation of the lockwire within its annular slot to thereby prevent escape of the lockwire from the rotor wheel by preventing rotation of the lockwire.